Silicon-on-sapphire (SOS) plays an important role as material especially suited for radiation-hard integrated circuits using complimentary metal oxide semiconductors (CMOS). The SOS technology has also been considered as a potential contributor to high performance very large scale integrated (VLSI) and very high scale integrated (VHSI) circuit applications. Recent work on SOS devices has clearly demonstrated a direct relationship between the crystalline perfection of SOS films and device parameters. It has also been shown that crystalline imperfections have a detrimental effect on electronic properties of SOS films, such as excess carrier lifetime, trapping centers, degree of amorphization and microscopic electrical inhomogeneities.
The pace of the optimization of the properties of materials in the heteroepitaxial silicon technology is closely related to the ability to characterize the quality of the deposits. The entire thickness of the film can be observed by cross-section transmission microscopy, but this method is laborious and time consuming, and therefore cannot serve as a rapid feed-back for material optimization efforts. UV reflectometry now serves as a rapid method to characterize the crystalline quality of the heteroepitaxial silicon surface most remote from the silicon/sapphire interface. See U.S. Pat. Nos. 4,352,016 and 4,352,017 issued on Sept. 22, 1982 to M. L. Duffy, et al. for a description of the use of UV reflectivity for determining the quality of silicon layers.
One method for the characterization of the crystalline nature of the silicon near or at the silicon/sapphire interface is described in our co-pending application Ser. No. 439,562 filed Nov. 5, 1982, now U.S. Pat. No. 4,498,772, issued Feb. 12, 1985, entitled "Method to Determine the Crystalline Properties of an Interface of Two Materials by an Optical Technique." According to that method, we utilize interference reflectivity phenomenon generated between the silicon/sapphire interface and measure reflections from the illuminated silicon surface. The measured reflections are used to determine the refractive index of the silicon at or near the interface which is a measure of the crystalline quality of the silicon. The index of refraction of the silicon in bulk form is different from the index of refraction of a thin film of silicon, particularly at or near the interface with sapphire.
We have now discovered that the crystalline quality of the silicon/sapphire interface can be characterized by photovoltage phenomenon. See U.S. Pat. No. 4,333,051, entitled "Method and Apparatus for Determining Minority Carrier Diffusion Length in Semiconductors," issued on June 1, 1982 to A. M. Goodman, for a description of using surface photovoltage phenomena to determine crystalline quality in terms of the minority carrier diffusion length. See also, U.S. patent application Ser. No. 619,368 of J. I. Pankove, et al. filed June 11, 1984, entitled "Method and Apparatus for Generating a Map Indicating the Quality of Semiconductor Material," for a description of using single wavelength surface photovoltage measurements directly to provide a map indication of the quality of the body of semiconductor material.